Hologram recording and reproducing device and method for recording hologram

ABSTRACT

A hologram recording and reproducing device controls an optical beam output from an external cavity semiconductor laser to improve a diffraction efficiency. The hologram recording and reproducing device includes an external cavity laser, a photodiode, a laser drive circuit and a laser diode controller. The external cavity laser has a laser diode adapted to emit an optical beam that is used to generate data light and reference light with which a hologram recording medium is irradiated. The photodiode detects the amount of the optical beam output from the external cavity laser. The laser drive circuit supplies a current to the external cavity laser. The laser diode controller controls the laser drive circuit to ensure that a value obtained by integrating the detected intensity of the optical beam with respect to time over a predetermined period is equal to predetermined recording energy.

The subject matter of application Ser. No. 12/509,674 is incorporatedherein by reference. The present application is a Continuation of U.S.application Ser. No. 12/509,674, filed Jul. 27, 2009, which claimspriority to Japanese Patent Application JP 2008-217739 filed in theJapanese Patent Office on Aug. 27, 2008, the entire contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hologram recording and reproducingdevice and a method for recording a hologram.

2. Description of the Related Art

In recent years, a hologram recording and reproducing device hasattracted attention as a data storage device. To record a hologram,reference light and data light are generated from laser light outputfrom a single light source. In this case, the data light is modulated onthe basis of recorded data. A hologram recording medium is irradiatedwith the reference light and the data light. The reference light and thedata light interfere with each other in the hologram recording medium toform the hologram (diffraction grating). The hologram is recorded in thehologram recording medium. The recorded hologram includes a large amountof information. To reproduce the recorded information from the hologramrecording medium, the hologram is irradiated with the reference light toensure that reproduction light (diffracted light) is generated. Thereproduction light is received by an imager having light receivingelements two-dimensionally arranged. Then, signal processing isperformed on the reproduction light to reproduce the recorded data.

In the aforementioned case, the data light and the reference light aregenerated by an optical unit having optical elements, and thereproduction light is received by the optical unit. A certain opticalunit has a common optical path through which data light and referencelight pass. That is, the certain optical unit has a coaxial interferencesystem (refer to Japanese Patent Publication No. 2003-178484). Inaddition, another optical unit has an optical path through which datalight passes and an optical path through which reference light passes.The two optical paths are different from each other. That is, theoptical unit has a two-beam interference system. As an optical sourcethat is adapted to generate an optical beam and used in a hologramrecording and reproducing device, there is a proposed technique relatedto an external cavity laser that emits a blue optical beam (refer toJapanese Patent Publication No. 2006-177995). In addition, there is aproposed technique for recording a hologram under the condition that ahologram recording medium moves (refer to Japanese Patent PublicationNo. 2007-178780). Furthermore, recording and reproducing characteristicsobtained when an external cavity semiconductor laser is used have beenreported (refer to “T. Tanaka, K. Takahashi, K. Sato, R. Kasegawa, M.Toishi, K. Watanabe, D. Samuels, and M. Takeya, “Littrow-typeexternal-cavity blue laser for holographic data storage,” Appl. Opt. 46,3583-3592 (2007)”).

SUMMARY OF THE INVENTION

There has not been a thorough study on how an external cavitysemiconductor laser is controlled in order to maintain a highdiffraction efficiency of reproduction light generated from a formedhologram in the case where the external cavity semiconductor laser isused in a hologram recording and reproducing device.

The present invention provides a hologram recording and reproducingdevice for controlling an optical beam output from an external cavitysemiconductor laser to improve a diffraction efficiency and a method forrecording a hologram with an improved diffraction efficiency.

The hologram recording and reproducing device according to an embodimentof the present invention includes: an external cavity laser having alaser diode adapted to emit an optical beam that is used to generatedata light and reference light with which a hologram recording medium isirradiated; a photodiode for detecting the amount of the optical beamemitted by the laser diode; a laser drive circuit for supplying acurrent to the laser diode; and a laser diode controller for controllingthe laser drive circuit to ensure that a value obtained by integratingthe detected intensity of the optical beam with respect to time over apredetermined period is equal to predetermined recording energy.

The method for recording a hologram according to an embodiment of thepresent invention includes the steps of: emitting, by means of a laserdiode included in an external cavity laser, an optical beam that is usedto generate data light and reference light with which a hologramrecording medium is irradiated; detecting the amount of the optical beamemitted by the laser diode by means of a photodiode; supplying a currentto the laser diode by means of a laser drive circuit; and controlling,by means of a laser diode controller, the laser drive circuit to ensurethat a value obtained by integrating the detected intensity of theoptical beam with respect to time over a predetermined period is equalto predetermined recording energy.

According to the embodiment of the present invention, the laser diodecontroller controls the laser drive circuit to ensure that the valueobtained by integrating the detected of the optical beam with respect totime over the predetermined period is equal to the predeterminedrecording energy. Thus, the length of time to record a hologram in ahologram recording medium can be a predetermined value. In addition, theamount of the recording energy applied to the hologram recording mediumcan be a predetermined value.

According to the embodiment of the present invention, a hologram isrecorded in the predetermined time period, and the predeterminedrecording energy is applied to the hologram recording medium. As aresult, a high diffraction efficiency of the reproduction lightgenerated from the formed hologram can be maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an optical unit that has a coaxialinterference system and is included in a hologram recording andreproducing device;

FIG. 2 is a schematic diagram showing the structure of a hologram disc;

FIG. 3 is a schematic diagram showing the hologram recording andreproducing device;

FIG. 4 is a diagram showing the relationship between opening and closingstates of a shutter and a swing angle of a galvanometer mirror;

FIG. 5 is a diagram showing the configuration of an external cavitylaser used in a recording laser according to an embodiment of thepresent invention;

FIGS. 6A to 6C are diagrams each showing an oscillation wavelengthspectrum of the recording laser;

FIG. 7 is a schematic diagram showing a variation in output power of theoptical beam output from the recording laser due to a mode hop;

FIG. 8 is a picture of the screen of an oscilloscope displaying thevariation in the optical power due to the mode hop;

FIG. 9 is a diagram showing the configuration of the recording laseraccording to the embodiment;

FIG. 10 is a flowchart of control of the recording laser according tothe embodiment; and

FIG. 11 is a flowchart of control of another recording laser accordingto the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

The following describes the best mode (embodiment) for carrying out theinvention. The description is provided in the following order. (1) Acoaxial interference system included in a hologram recording andreproducing device. (2) The structure of a hologram disc. (3) An actionof an optical beam in the hologram disc. (4) The hologram recording andreproducing device. (5) Synchronization of an operation of agalvanometer mirror with opening and closing of a shutter. (6) Anexternal cavity laser. (7) A recording laser according to the embodimentof the present invention. (8) A method for controlling the recordinglaser according to the embodiment.

The term of the “hologram recording and reproducing device” used in thefollowing description includes three meanings. The first meaning is ahologram recording device that records a hologram in a hologram disc.The second meaning is a hologram reproducing device that reproducesinformation from the hologram recorded in the hologram disc. The thirdmeaning is a hologram recording and reproducing device that records ahologram in a hologram disc and reproduces the hologram recorded in thehologram disc.

(Coaxial Interference System Included in Hologram Recording andReproducing Device)

FIG. 1 is a schematic diagram showing an optical unit 50 that is thecoaxial interference system included in the hologram recording andreproducing device. The optical unit 50 has a recording/reproducingoptical system (blue laser system) for recording and reproducingholographic data in and from a hologram disc 1. The hologram disc 1 is ahologram recording medium. The optical unit 50 also has a servo opticalsystem (red laser system). The servo optical system specifies a regionof the hologram disc 1 in order to record a hologram in the region ofthe hologram disc 1 and positions an optical beam in the specifiedregion. In addition, the servo optical system specifies a region of thehologram disc 1 in order to reproduce a hologram from the region of thehologram disc 1 and positions an optical beam in the specified region.

The blue laser system is an optical system that includes optical membersadapted to transmit or reflect a blue laser beam (blue optical beam).The blue optical beam is used to record and reproduce a hologram. Therecording laser is denoted by reference numeral 7. The blue optical beamis output from the recording laser 7. The red laser system is an opticalsystem that includes optical members adapted to transmit or reflect ared laser beam (red optical beam). The red optical beam is used for aservo operation. The red optical beam is output from a servo laser 4.The recording laser 7 has a characteristic configuration specific to theembodiment, and is described later in detail.

Parts of the optical members are adapted to transmit or reflect both theblue optical beam and the red optical beam. The optical members thattransmit or reflect both the blue optical beam and the red optical beamare a dichroic prism 6, a λ/4 plate (¼ wavelength plate) 18 and anobjective lens 3. The blue optical beam and the red optical beam passthrough a single optical path or are reflected in the single opticalpath. Thus, the servo operation (positioning operation) can be performedby means of the red optical beam, while a hologram can be recorded andreproduced in and from the hologram disc 1 by means of the blue opticalbeam. The hologram disc 1 has a recording layer 32 (refer to FIG. 2).Optical characteristics of the recording layer 32 are not changed by thered optical beam. Therefore, recording and reproducing characteristicsof the hologram disc 1 are not affected by the servo operation.

The servo optical system has the following optical members, and the redoptical beam is transmitted or reflected by each of the optical membersin the following way. The red optical beam output from the servo laser 4is divided into three optical beams by a grating 5. The three opticalbeams pass through a beam splitter 15 and are reflected by the dichroicprism 6. The three optical beams are then incident on the objective lens3. The objective lens 3 focuses the optical beams to irradiate thehologram disc 1 with the focused optical beams. In this case, the servooptical system operates in order to focus the optical beams on apredetermined region of the hologram disc 1. The servo optical system isdescribed later.

The recording/reproducing optical system (blue laser system) has thefollowing optical members, and the blue optical beam is transmitted orreflected by each of the optical members in the following way. The blueoptical beam output from the recording laser 7 passes through anisolator 8, a shutter 9 and the like, and is incident on a galvanometermirror 10. The galvanometer mirror 10 scans the blue optical beam. Thescanned blue optical beam is reflected by a polarizing beam splitter(PBS) 12 and incident on a spatial modulator 11. The spatial modulator11 modulates the incident blue optical beam to generate data light andreference light. The data light and the reference light pass through thePBS 12. Then, the data light and the reference light pass through arelay lens 16, a polarizing beam splitter (PBS) 14 and a relay lens 17and are incident on the dichroic prism 6.

The dichroic prism 6 has an optical film formed thereon to reflect thered optical beam and transmit the blue optical beam. Thus, the blueoptical beam passes through the dichroic mirror 6. The blue optical beampasses through the λ/4 plate 18 and is focused by the objective lens 3.The hologram disc 1 is then irradiated with the blue optical beam. Thedata light and the reference light interfere with each other in therecording layer 32 (refer to FIG. 2) to ensure that a hologram isrecorded in the recording layer 32.

(Structure of Hologram Disc)

FIG. 2 is a schematic diagram showing the structure of the hologram disc1. The hologram disc 1 has an antireflective film 30, a plasticsubstrate 31, the recording layer 32, a gap layer 33, a wavelengthselection film 34, a gap layer 35 and a plastic substrate 36, which arelaminated. The plastic substrate 31 has a thickness of 0.6 millimeters.The recording layer 32 has a thickness of 0.6 millimeters as a typicalexample. The gap layer 33 has a thickness of 0.1 millimeters as atypical example. The gap layer 35 has a thickness of 0.1 millimeters asa typical example. Address grooves, track grooves and the like areprovided on an interface between the gap layer 35 and the plasticsubstrate 36. An aluminum reflective film is provided on the interfaceon which the address grooves, the track grooves and the like areprovided. The aluminum reflective film, the address grooves and thetrack grooves constitute the interface and are collectively denoted byreference numeral 37 as shown in FIG. 2.

(Action of Optical Beam in Hologram Disc)

A servo beam 38 shown in FIG. 2 schematically indicates a single redoptical beam. The hologram disc 1 is irradiated with the red opticalbeam coming from the objective lens 3. The servo beam 38 passes throughthe antireflective film 30, the plastic substrate 31, the recordinglayer 32 and the gap layer 33. The servo beam 38 then reaches thewavelength selection film 34. The wavelength selection film 34 transmitsor reflects an optical beam depending on the wavelength of the opticalbeam. The wavelength selection film 34 transmits the red optical beam.Thus, the servo beam 38, which is the red optical beam, passes throughthe wavelength selection film 34. The servo beam 38 then passes throughthe gap layer 35 and is reflected on the interface 37 (constituted bythe aluminum reflective film, the track grooves and the addressgrooves). In this case, the servo beam 38 is modulated on the basis ofthe shapes of the address and track grooves formed on the interface 37(constituted by the aluminum reflective film, the track grooves and theaddress grooves) and retrieves servo information.

The servo beam 38 reflected on the interface 37 (constituted by thealuminum reflective film, the track grooves and the address grooves)passes through the gap layer 35, the wavelength selection film 34, thegap layer 33, the recording layer 32, the plastic substrate 31 and theantireflective film 30 and reaches the objective lens 3. The servo beam38 coming from the objective lens 3 is reflected by the dichroic prism6, reflected by the beam splitter 15, and incident on a photodetector(PD) 19. The photodetector 19 obtains a servo error signal that issimilar to a servo error signal detected from an optical disc such as adigital versatile disc. For example, a focus error signal can beobtained by an astigmatic method, and a tracking error can be obtainedby a push-pull method.

A recording beam 39 shown in FIG. 2 schematically indicates a singleblue optical beam. The recording beam 39 is the blue optical beam comingfrom the objective lens 3 and includes the data light and the referencelight. The hologram disc 1 is irradiated with the recording beam 39. Therecording beam 39 then passes through the antireflective film 30 and theplastic substrate 31 and reaches the recording layer 32. The data lightand the reference light interfere with each other in the recording layer32 to form an interference fringe. The interference fringe is recordedas a hologram.

To reproduce the hologram, the reference light travels in the same pathas that of the recording beam 39, and the hologram disc 1 is irradiatedwith the reference light as shown in FIG. 2. The reference light thenpasses through the antireflective film 30 and the plastic substrate 31and reaches the recording layer 32. The hologram recorded in therecording layer 32 is irradiated with the reference light to ensure thatreproduction light (diffracted light) is generated. Since thereproduction light is a blue optical beam, the reproduction light isreflected by the wavelength selection film 34, and retrieves informationrecorded in the hologram.

The reproduction light reflected by the wavelength selection film 34passes through the gap layer 35, the wavelength selection film 34, thegap layer 33, the recording layer 32, the plastic substrate 31 and theantireflective film 30 and reaches the objective lens 3. Thereproduction light coming from the objective lens 3 is reflected by thepolarizing beam splitter 14 and incident on a complimentary metal oxidesemiconductor (CMOS) imager 13 serving as an imaging device. Theinformation recorded in the hologram can be read by processing an imageof the reproduction light received by the CMOS imager 13.

(Hologram Recording and Reproducing Device)

FIG. 3 is a schematic diagram showing the hologram recording andreproducing device. The hologram recording and reproducing device has aspindle motor 20, a feed motor 21, a photodetector signal processingcircuit 22, a servo controller 23, an address decode timing generator24, a host controller 25 and an imager signal processor 29. The servocontroller 23 controls a servo system of the hologram recording andreproducing device. The host controller 25 transmits and receives asignal to and from the servo controller 23. The host controller 25transmits an instruction to the servo controller 23. In addition, thehost controller 25 transmits and receives a signal to and from theaddress decode timing generator 24. Furthermore, the host controller 25transmits and receives a signal to and from the optical unit 50.

The hologram disc 1 has a discoid shape and is mounted on a rotary shaftof the spindle motor 20. The servo controller 23 controls a rotation ofthe spindle motor 20. As a result, a rotation of the hologram disc 1 iscontrolled by the servo controller 23. The servo controller 23 drives aservo actuator 2 to move the objective lens 3 in a focusing directionand in a tracking direction. In addition, the servo controller 23 movesthe feed motor 21 in the tracking direction to perform a tracking servooperation in a range that is not covered by the movement of theobjective lens 3.

The imager signal processor 29 processes the reproduction light receivedby the optical unit 50 and reproduces the data included in thereproduction light. The imager signal processor 29 transmits the data tothe host controller 25. The photodetector 19 included in the opticalunit 50 receives the red optical beam and generates a PD signal 26. Thephotodetector 19 transmits the PD signal to the PD signal processingcircuit 22. The PD signal processing circuit 22 processes the PD signal26. One piece of the information retrieved by the red optical beam is atrack reproduction signal 28. The PD signal processing circuit 22transmits the track reproduction signal 28 to the address decode timinggenerator 24. The address decode timing generator 24 then decodesaddress information from the received track reproduction signal 28 tospecify a region of the interface 37 irradiated with the red opticalbeam. Specifying the region of the interface 37 irradiated with the redoptical beam specifies a region of the recording layer 32 irradiatedwith the blue optical beam. The PD signal processing circuit 22 suppliesa servo error signal 27 to the servo controller 23. The servo errorsignal 27 is another piece of the information retrieved by the redoptical beam.

The address decode timing generator 24 transmits the decoded addressinformation to the host controller 25. The host controller 25 receivesthe address information and outputs the address information to the servocontroller 23. The servo controller 23 receives the address information.The servo controller 23 then controls each actuator on the basis of theaddress information to direct the red and blue optical beams to thespecified region of the recording layer 32. The host controller 25transmits and receives recording and reproducing data to and from theoptical unit 50 to perform a recording operation and a reproducingoperation. The host controller 25 receives laser power information(refer to FIG. 9) from the optical unit 50 and outputs laser controlinformation (refer to FIG. 9) and an ON/OFF signal (refer to FIG. 9) tothe optical unit 50.

The outline of the configuration and operations of the hologramrecording and reproducing device is described above. The followingdescribes main parts of the hologram recording and reproducing deviceaccording to the embodiment in detail.

(Synchronization of Operation of Galvanometer Mirror with Opening andClosing of Shutter)

Recording of holographic data is different from recording of data on anormal optical disc. To record holographic data, a page recording methodis used. In the page recording method, data having a constant amount isrecorded at one time. The galvanometer mirror 10 scans the blue opticalbeam to irradiate a certain region of the hologram disc 1 for a certaintime under the condition that the hologram disc 1 is continuouslyrotated by means of the spindle motor 20. This technique is described inJapanese Patent Publication No. 2007-178780.

Only during the scanning of the blue optical beam by the galvanometermirror 10, the shutter 9 is open to transmit the blue optical beam fromthe isolator 8 to the galvanometer mirror 10. The shutter 9 is closed toprevent the blue optical beam from reaching the galvanometer mirror 10after the scanning is terminated and before the galvanometer mirror 10returns to a position at which galvanometer mirror 10 performs the nextscanning. In this case, the servo controller 23 controls the shutter 9to ensure that the shutter 9 is closed.

FIG. 4 shows the relationship of an opening state (in which the blueoptical beam passes through the shutter 9) and closing state (in whichthe blue optical beam does not pass through the shutter 9) of theshutter 9 and a swing angle of the galvanometer mirror 10.

The hologram is recorded in the recording layer 32 of the hologram disc1 in the following manner. The spatial modulator 11 displays a patternin which the blue optical beam reflected by the spatial modulator 11 isrepresented by the data light and the reference light. The hostcontroller 25 controls the spatial modulator 11 to ensure that thespatial modulator 11 displays the pattern.

To reproduce the hologram, the spatial modulator 11 displays a patternin which the blue optical beam reflected by the spatial modulator 11 isrepresented only by the reference light. The host controller 25 controlsthe spatial modulator 11 to ensure that the spatial modulator 11displays the pattern. As described above, when the hologram disc 1 isirradiated with the reference light, the reproduction light is generatedfrom the recorded hologram and incident on the CMOS imager 13. In thiscase, the galvanometer mirror 10 and the shutter 9 are synchronouslycontrolled as shown in FIG. 4 in the same manner as the synchronizedoperations performed during the recording.

(External Cavity Laser)

FIG. 5 is a diagram showing the configuration of an external cavitylaser 57 used in the recording laser 7 according to the embodiment. Asthe external cavity laser 57, a Littrow external cavity laser is used.The external cavity laser 57 includes a laser diode 40, a collimate lens41, and a reflective diffraction grating 42. The laser diode 40 emits ablue optical beam. The blue optical beam emitted by the laser diode 40is converted into parallel light by the collimate lens 41. The parallellight is then incident on the reflective diffraction grating 42. Whenthe parallel light is reflected by the reflective diffraction grating42, the parallel light is separated into zero order light 44 and primarylight 43. The primary light 43 reflected and diffracted by thereflective diffraction grating 42 passes through the collimate lens 41again and returns to the laser diode 40 as indicated by an arrow shownin FIG. 5. Due to the returned optical beam, a cavity is formed betweenthe reflective diffraction grating 42 and the laser diode 40. The laserdiode 40 oscillates at a wavelength determined on the basis of an angleof incidence of the light on the reflective diffraction grating 42. Thezero order light 44 is reflected in a similar way to reflection on anormal mirror and diffracted by the reflective diffraction grating 42.The zero order light 44 is then output from the external cavity laser 57as an optical beam to be used to record or reproduce a hologram.

FIGS. 6A to 6C are diagrams each showing an oscillation wavelengthspectrum of the external cavity laser 57 that constitutes a part of therecording laser 7. An oscillation wavelength spectrum of the opticalbeam emitted by the external cavity laser 57 indicates that the laserdiode 40 oscillates at a substantially single wavelength in many cases.However, an oscillation mode of the laser diode 40 changes depending onthe value of a current supplied to the laser diode 40 of the externalcavity laser 57 and on an ambient temperature of the laser diode 40.FIG. 6A shows an oscillation mode in the case where the laser diode 40oscillates at multiple wavelengths. FIG. 6B shows an oscillation mode inthe case where the laser diode 40 oscillates at a single wavelength.FIG. 6C shows an oscillation mode in the case where the laser diode 40oscillates at multiple wavelengths including two peak wavelengths. Thelaser diode 40 may oscillate in any of the oscillation modes shown inFIGS. 6A to 6C depending on the value of the current supplied to thelaser diode 40 and on the ambient temperature of the laser diode 40. Amode hop means a high-speed transition between any two of the threewavelengths in the oscillation mode shown in FIG. 6A and a high-speedtransition between any two of the six wavelengths in the oscillationmode shown in FIG. 6C.

FIG. 7 is a graph schematically showing a variation in output power ofthe optical beam output from the recording laser 7. The output power isplotted along an ordinate axis of the graph, while time is plotted alongan abscissa axis of the graph. An experiment has confirmed that opticalpower of the zero order light output from the external cavity laser 57varies up to approximately 7 percent. The variation of 7 percent in theoptical power causes deficient exposure or excessive exposure forrecording of a hologram. FIG. 8 is a picture of the screen of anoscilloscope displaying the variation in the optical power. As shown inFIG. 8, the variation in the optical power occurs for an extremely shorttime, e.g., 1 microsecond or less. It is difficult to suppress thevariation in the optical power by means of a control circuit.

(Recording Laser According to the Embodiment)

FIG. 9 is a diagram showing the configuration of the recording laser 7according to the embodiment. The recording laser 7 shown in FIG. 9includes the littrow external cavity laser 57 (shown in FIG. 5), a beamsplitter 51, a photodiode 52, a laser power monitor amplifier 53, ananalog-to-digital (AD) converter 56, a digital-to-analog (DA) converter58, and a laser drive circuit 55. The laser drive circuit 55 supplies acurrent to the external cavity laser 57 that is one type of laser diode.The host controller 25 serves as a laser diode controller forcontrolling the laser drive circuit 55. The laser drive circuit 55 isdriven by means of an analog signal. Since the host controller 25outputs a digital signal, the host controller 25 controls the laserdrive circuit 55 via the DA converter 58.

The optical beam (zero order light) 44 output from the external cavitylaser 57 is divided into two beams by the beam splitter 51. One of thetwo beams propagates straight and is used to record a hologram. Theother of the two beams is reflected by the beam splitter 51 and incidenton the photodiode 52. The photodiode 52 then outputs to the laser powermonitor amplifier 53 a current whose amount is proportional to theamount of the optical beam incident on the photodiode 52. That is, thephotodiode 52 detects the amount of the optical beam emitted by thelaser diode. The laser power monitor amplifier 53 converts the currentinto a voltage. The voltage output from the laser power monitoramplifier 53 is proportional to the power of the optical beam detectedby the photodiode 52 and proportional to the power of the optical beamto be used to record the hologram.

The AD converter 56 converts an analog value of the voltage output fromthe laser power monitor amplifier 53 into a digital value, and outputsthe digital value to the host controller 25. The host controller 25performs a calculation on the basis of the laser power informationreceived from the AD converter 56 and on other information (e.g.,information on an environmental temperature detected by a temperaturesensor (not shown)) and outputs to the DA converter 58 the laser controlinformation suitable for recording and reproducing the hologram. Thehost controller 25 outputs the ON/OFF signal to the laser drive circuit55 to instruct the laser drive circuit 55 to supply a laser drive signalto the external cavity laser 57 and stop supplying the laser drivesignal and thereby control the external cavity laser 57 to ensure thatthe external cavity laser 57 outputs the optical beam and stopsoutputting the optical beam. As described above, the red optical beam isused for the servo operation. Thus, the servo operation is maintainedeven when the external cavity laser 57 stop outputting the optical beam.When the shutter 9 is closed under the condition that the externalcavity laser 57 outputs the optical beam, the blue optical beam does notreach the hologram disc 1. Even in this case, the servo operation ismaintained.

(Method for Controlling Recording Laser According to the Embodiment)

As described above, the blue optical beam is divided into the referencelight and the data light modulated on the basis of recorded data. Thehologram disc 1 is irradiated with the reference light and the datalight. The reference light and the data light interfere with each otherin the recording layer 32 to form an interference fringe. Theinterference fringe is recorded as a hologram. As the recording layer32, photosensitive monomer is used. The monomer is converted to polymerto form the hologram. The recorded hologram is reproduced by irradiatingthe hologram only with the reference light. The higher a diffractionefficiency determined on the basis of the quality of the recordedhologram, the greater a signal level of the reproduction light.

The diffraction efficiency is determined on the basis of energy(recording energy) of the optical beam with which the hologram disc 1 isirradiated for the recording of the hologram. The recording energy isrepresented by integrating laser power (that is the amount of therecording light) for the recording with respect to time. It is thereforenecessary that the recording energy be controlled to ensure that thediffraction efficiency is optimal or maximal. The value of the recordingenergy when the diffraction efficiency is maximal is referred to as theoptimal recording energy value.

The optimal recording energy value varies depending on a property of themedium having the recording layer 32 formed therein. However, theoptimal recording energy value can be specified by specifying the typeand temperature of the medium. In addition, the optimal recording energyvalue is represented by a value (integrated amount) obtained byintegrating the intensity of the recording light with respect to time.The optimal recording energy value can be obtained by setting the amountof the integrated intensity to a pre-specified value. The following twomethods are used to provide the optimal recording energy to therecording layer 32.

First Method

The intensity of the recording light used after the start of therecording is integrated. When the amount of the integrated intensity ofthe light reaches a preset level (corresponding to the optimal recordingenergy), the recording stops.

Second Method

The recording time (for which the recording layer 32 is irradiated withthe reference light and the data light) is regarded as a predeterminedtime period that is a constant preset value. The power of the blueoptical beam for the recording is controlled to ensure that the amountof the integrated intensity of the light reaches predetermined recordingenergy (referred to as the optimal recording energy) at the time oftermination of the recording. The optimal recording energy is a constantpreset value.

The recording time is controlled in the first method, while the amountof the recording light (which is the power of the blue optical beam) iscontrolled in the second method. In order to record a hologram in thehologram disc rotating at a constant rate, it is preferable that theopening and closing of the shutter, the galvanometer mirror, and therotation of the hologram disc be synchronized. From the perspective ofthe synchronization, the second method is more suitable than the firstmethod. The following describes the case where the second method isused.

The following formulas (1) and (2) are established, where Ew isremaining recording energy; Tw is a remaining exposure time; Pw isrecording power (power of the optical beam); and Ts is a controlinterval (sampling time (cycle)).Ew=Ew−1−(Pw−1×Ts)  Formula 1

The symbol Ew−1 indicates remaining recording energy at the time of theprevious sampling, while the symbol Pw−1 indicates recording power atthe time of the previous sampling.Tw=Tw−1−Ts  Formula 2

The symbol Tw−1 indicates a remaining exposure time at the time of theprevious sampling.

As a formula to represent recording energy to be set for the currentsampling cycle, the following formula (3) is established. That is, whena hologram is recorded with the recording energy represented by theformula (3) in the remaining exposure time, the recording can beterminated in the predetermined time period, and the predeterminedrecording energy can be provided to the hologram disc 1.Pw=Ew/Tw  Formula 3

When the power of the optical beam represented by the formula (3) isused to represent the recording power used in the current sampling cycleand remaining sampling cycles, the recording power can be modified. Inthis way, the hologram can be recorded with the maximal diffractionefficiency.

FIG. 10 is a flowchart of control of the recording laser according tothe embodiment.

In step ST100, the host controller 25 sets, as an initial value of theremaining recording energy Ew, the recording energy (optimal recordingenergy) that leads to the maximal diffraction efficiency of the usedrecording medium (recording layer 32 of the hologram disc 1).

The optimal recording energy value is obtained through an experiment inadvance.

In step ST101, the host controller 25 sets, as an initial value of theremaining exposure time Tw, the predetermined time period for which theused recording medium (recording layer 32 of the hologram disc 1) isirradiated with the optical beam. The predetermined time period isdetermined on the basis of the rotation rate of the spindle motor 20 andan interval between times when holograms are recorded.

In step ST102, the host controller 25 controls the ON/OFF signal toensure that the recording laser 7 outputs the optical beam.

In step ST103, the host controller 25 calculates the formula (3) toobtain the recording power Pw.

In step ST104, the host controller 25 transmits a laser control signalto the DA converter 58 on the basis of the calculation result obtainedin step ST103 and sets the recording power P.w.

In step ST105, the host controller 25 determines whether or not thevalue of (Tw−Ts) is equal to or less than zero. When the host controller25 determines that the value of (Tw−Ts) is equal to or less than zero(or when the answer is Yes in step ST105), the process shown in FIG. 10proceeds to step ST108. When the host controller 25 determines that thevalue of (Tw−Ts) is larger than zero (or when the answer is No in stepST105), the process shown in FIG. 10 proceeds to step ST106.

In step ST106, the host controller 25 calculates the remaining recordingenergy Ew. The remaining recording energy Ew is reduced from the initialvalue set in step ST100 when the process shown in FIG. 10 passes throughstep ST106. The process then proceeds to step ST107.

In step ST107, the host controller 25 calculates the remaining exposuretime Tw. The remaining exposure time Tw is reduced from the initialvalue set in step ST101 when the process shown in FIG. 10 passes throughstep ST107. The process then proceeds to step ST103.

In step ST108, the host controller 25 turns off the recording laser 7 toprevent the hologram disc 1 from being irradiated with the optical beamoutput from the recording laser 7. The process is then terminated. Theirradiation of the recording layer 32 with the blue optical beam can bestarted by turning on the recording laser 7 in step ST102. Theirradiation of the recording layer 32 with the blue optical beam can bestopped by turning off the recording laser in step ST108. Thus, therecording time can be controlled without the shutter 9 in the processshown in FIG. 10.

A timer (not shown) or the like is provided in the host controller 25.The timer or the like is set to ensure that a time interval for which aloop of steps ST103 to ST107 is performed is equal to the sampling timeTs. The sampling time Ts is sufficiently smaller than a time necessaryto record a hologram. The host controller 25 may perform the calculationprocessing in step ST103 when a timer interrupt occurs for each samplingtime Ts, instead of using the timer or the like. The time length betweenthe time when step ST103 is first performed and the time when step ST108is performed is equal to the recording time (for which a hologram isrecorded in the hologram disc 1) that is the predetermined time period.For example, the recording time is set equal to a time (shown in FIG. 4)for which the shutter 9 is in an opening state.

FIG. 11 is a flowchart of control of another recording laser accordingto the embodiment. In the flowchart shown in FIG. 11, step ST1021 isperformed instead of step ST102 of the flowchart shown in FIG. 10, andstep ST1081 is performed instead of step ST108 of the flowchart shown inFIG. 10. The other steps shown in FIG. 11 are the same as those shown inFIG. 10, and thus not described below.

In step ST1021, the shutter 9 is open. This results in the fact that therecording layer 32 is irradiated with the blue optical beam. In stepST1081, the shutter 9 is closed. This results in the fact that theirradiation of the recording layer 32 with the blue optical beam isstopped. In normal hologram recording, several hundred page data pieces(holograms) or more are recorded. In this case, one page data pieceforms one hologram. As shown in the flowchart of FIG. 11, the recordinglaser 7 is turned on in order to start the recording, and the shutter 9is open and closed for each page to record a hologram. After a series ofoperations for recording holograms is terminated, the recording laser 7is turned off.

The control described above is summarized as follows. The remainingrecording energy value and the remaining exposure time are calculatedfor each sampling time Ts. The recording power is determined on thebasis of the calculated remaining recording energy value and thecalculated remaining recording time, and the recording is performed withthe determined recording power. Thus, the optimal recording can beperformed in the predetermined recording time.

In general, any of the three oscillation modes shown in FIGS. 6A to 6Cmay occur in the external cavity laser 57 when the laser power isadjusted as described above. The oscillation mode shown in FIG. 6A isreferred to as a single-oscillation mode. The oscillation mode shown inFIG. 6B is referred to as a three-oscillation mode. The oscillation modeshown in FIG. 6C is referred to as a six-oscillation mode. In the lasercontrol method according to the embodiment, the laser power is adjustedwhile the recording time is fixed. Thus, any of the three oscillationmodes (single-oscillation mode, three-oscillation mode andsix-oscillation mode) may occur.

When a coaxial interference method is used, a hologram can beexcellently recorded in any of the three oscillation modes. When atwo-beam interference method is used, a hologram can be recorded in thesingle-oscillation mode and the three-oscillation mode. However, theaforementioned document, Littrow-type external cavity blue laser forholographic data storage, describes that a hologram may not be recordedin the six-oscillation mode when a two-beam interference method is used.According to the results of the experiment described in the presentspecification and conducted by the present inventors, when the length ofa chip of the laser diode 40 included in the Littrow external cavitylaser 57 is 1 millimeter or more and a two-beam interference method isused, a hologram can be recorded in a recording layer having a thicknessof 1.5 millimeters in the six-oscillation mode.

The laser control method according to the embodiment can be usedregardless of methods for recording holographic data including arecording method using a coaxial interference method and a recordingmethod using a two-beam interference method.

In the laser control method according to the embodiment, even when it isnecessary to specify the time necessary to record a hologram (forexample, even when the laser control is synchronized with the angularvelocity of the rotation of the spindle motor 20), the diffractionefficiency of the formed hologram is high. This feature is effective fora hologram recording and reproducing device that records a hologramunder the condition that a disc-type hologram medium rotates.

In the laser control method according to the embodiment, the power ofthe optical beam output from the recording laser varies. The amount ofthe integrated intensity of the optical beam output from the recordinglaser significantly affects the formation of a hologram. Since theoutput power of the optical beam varies for the sampling time Ts, thechange in the output power of the optical beam slightly affects theformation of the hologram. The laser diode easily transitions betweenany two of the oscillation modes shown in FIGS. 6A to 6C when the valueof a current to be supplied to the laser diode is changed in order tochange the output power of the optical beam. In this case, when acoaxial interference method is used, a high diffraction efficiency canbe obtained. Even when a two-beam interference method is used, a highdiffraction efficiency can be obtained by controlling the length of thechip of the laser diode.

The embodiment describes that the hologram recording medium is thehologram disc. In the laser control method according to the embodiment,a hologram recording medium having a square shape may be used.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2008-217739 filedin the Japan Patent Office on Aug. 27, 2008, the entire content of whichis hereby incorporated by reference.

The present invention is not limited to the aforementioned embodimentand may be modified in various ways within the scope of the technicalconcept of the invention.

1. A hologram recording and reproducing device comprising: a laser diodeadapted to emit an optical beam that is used to generate data light andreference light; a photodiode that detects an amount of lightcorresponding to the optical beam emitted by the laser diode; a laserdrive circuit that supplies a current to the laser diode; and a laserdiode controller that controls the laser drive circuit to ensure that avalue obtained by integrating the detected intensity of the optical beamwith respect to time over a predetermined period corresponds to a valuethat is equal to or greater than a predetermined recording energy. 2.The hologram recording and reproducing device according to claim 1,wherein the laser diode controller sets the predetermined recordingenergy as an initial value of remaining recording energy; sets thepredetermined time period as an initial value of a remaining exposuretime; calculates recording power by dividing the remaining recordingenergy by the remaining exposure time for each sampling time in order tosupply a current corresponding to the calculated recording power to thelaser diode; sets a value obtained by subtracting from the remainingrecording energy a value obtained by integrating the intensity of theoptical beam with respect to the sampling time as a new value of theremaining recording energy; sets a value obtained by subtracting thesampling time from the remaining exposure time as a new value of theremaining exposure time, and when the remaining exposure time becomeszero or less, the irradiation of the hologram recording medium with thedata light and the reference light is stopped.
 3. The hologram recordingand reproducing device according to claim 2, wherein the length of achip of the laser diode is 1 millimeter or more and set to allow thelaser diode to emit a blue optical beam.
 4. A method for recording ahologram comprising the steps of: emitting an optical beam that is usedto generate data light and reference light with which a hologramrecording medium is irradiated; detecting a signal corresponding to theamount of the optical beam emitted by the laser diode by means of aphotodiode; supplying a current to the laser diode by means of a laserdrive circuit; and controlling the laser drive circuit to ensure that avalue obtained by integrating the detected intensity of the optical beamwith respect to time over a predetermined period corresponding to avalue that is equal to or greater than a predetermined recording energy.